why are american politicians so freaking selfish and stupid?
Because the main way to get into high political office, for the most part, is your ability to win elections....the end, full stop.
We're ostensibly supposed to elect the "best of us", the most qualified and understanding and intelligent and prudent. However, it's not how qualified you are, or how intelligent, empathetic, and well-informed you are or your leadership ability...it's 95% about your ability to get people to vote for you, by hook or by crook. It's about how persuasive you are, how appealing you are, and how skilled you are at campaigning and manipulating cognitive biases and groupthink via the media and your speech and smile. Being able to convince others to vote for you (by any means necessary, even with half-truths or lies....so having a conscience might be a liability) leads to winning elections which leads to receiving more money...which leads to you being able to win more elections, and so on.
So to win high-level elections you need to be ultra-self-centered and image-obsessed, cutthroat (which requires a weak sense of empathy), and have a team of people who's sole purpose is to pump you up and make you more and more appealing to more and more people. A degree of psychopathy is helpful. The more people praise you and follow you and "believe" in you, the more you buy in to the narrative about yourself. That confidence, in turn, leads to you acting more electable...despite the fact that you may or may not actually be educated or qualified. Most people do not posses this level of narcissism or, again, psychopathy, or poopchute-ishness.
